How to Place the Right Seams for UV Unwrapping
In my opinion you should try to place the seams in that way, so the UV islands in UV/Image Editor are even distributed and have a proper scale that reflects the scale of the model's elements. It is often the matter of experimenting. You should think of it as it was a real model. Try to imagine how you may cut it open with scissors and spread on a flat surface. 


The example below shows how the different unwrap of the same object results in less uniform Uv islands distribution, which is less beneficial.


Here is the .blend with the seams placed: http://www.pasteall.org/blend/33446